Acoustic waves can be used to handle or sort particles in a fluid by means of an acoustic force field. In the conventional techniques, known in the prior art, acoustic resonators comprises a cavity wherein one of the walls comprises an acoustic wave generator and the opposing wall serves as passive reflector. The ultrasonic wave generated is reflected and the wave superposition is known as a standing wave: at least one acoustic pressure node is created at a given position along a dimension of a cavity of an acoustic resonator by providing a resonance condition for the acoustic wave. Particles manipulation with ultrasonic standing waves is known as a powerful tool for handling, moving or trapping particles in microfluidics devices.
For instance, U.S. Pat. No. 7,373,805 discloses an acoustic resonator comprising an ultrasonic transducer glued to a coupling plate, an acoustic resonator and a spacer arranged between the coupling plate and the acoustic resonator and defining a resonant cavity. The coupling plate, the spacer and the acoustic resonator are held in place between a base and a top plate secured with screws. The screws must be strongly tightened in order to limit leakages of fluid filling the resonant cavity. Furthermore, the screwing between the base and the top plate, which defined the thickness of the resonant cavity, must be precisely controlled in order to ensure a regular thickness.
Within acoustic resonator, the thickness of the resonant cavity is indeed of the utmost importance as the resonance frequency is achieved by reaching the following condition: w=n.
  λ  2wherein w is the thickness of the resonator, n is the number of pressure nodes of the standing waves and λ the wavelength equal to
      c    f    fwherein cf is the speed of sound in the fluid filling the resonant cavity of the resonator and f is the acoustic frequency.
Consequently, the resonator disclosed in U.S. Pat. No. 7,373,805 requires a complicated assembly and may exhibit leakages.
US patent application 2013/0327130 also discloses acoustic resonators comprising four layers: a transducer, a coupling steel layer, a spacer and an acoustic reflector. The spacer is assembled in sandwich between the coupling steel layer and the acoustic reflector with neoprene glue. Therefore, the resonator disclosed in US 2013/0327130 cannot be reused; especially the spacer cannot be changed in order to change the size of the resonant cavity.
There is thus a need for an easy-to-use acoustic resonator, avoiding leakage of fluid and wherein the resonant cavity may be opened and accessible and wherein the spacer may be easily changed in order to change the size of the resonant cavity. Especially, as the device of the invention may be used with living objects, there is a particular need for a device which may be easily disassembled and cleaned, e.g. autoclaved.
Moreover, the acoustic resonators of the prior art are designed for handling only a small amount of particles—from 1 to several hundred—within microfluidic devices. U.S. Pat. No. 7,373,805 discloses indeed the use of particles of diameter in the order of 1 μm and US 2013/03271430 discloses that the particles have an average size of about 50 nm to about 5 μm. There is therefore also a need for devices suitable for the analysis, separation and collection, without complex manipulation, of large particles, especially large living particles such as cells or cluster of cells, having an average size from 0.1 μm to few hundreds micrometers.